Dry etching uses a low pressure glow discharge to remove a thin solid surface deposition, be it of a film on a workpiece, as on a wafer, as a part of a production procedure of such workpiece or from a reactor vessel for cleaning purposes. The glow or plasma discharge is supplied with a given flow of an etching gas. The etching gas molecules are thereby rich of an etching element, such as of Fluorine, sometimes Chlorine or even more complex species. Upon dissociation of the etching gas in the discharge, etching radicals are formed. These radicals react on the surface with the depositions thereon to be removed to turn it into a gaseous product which is then removed from the reactor by pumping.
Other gases are usually added to the etching gas in order to optimise the etching process, such as oxygen as shown in table 1.
There are many reasons for such additive gases, as e.g.:
limiting the contamination of the reactor by etching by-products, as e.g. by S from SF.sub.6 etching gas; PA1 improving the selectivity for etching different material surfaces, as e.g. one film to be etched and one to remain unetched; PA1 adjusting the relative etch rate for two different material surfaces to be etched, as e.g. of two films; PA1 improving uniformity of the etching rate along the surface to be treated. PA1 The reactor walls for the second and for subsequent runs are different from the previous runs, due to reactor wall coating. Thus, the properties of the coating deposited on the substrate may vary from run to run. PA1 The coatings especially deposited in PECVD are usually hard and brittle. Hence, such films deposited on the reactor walls, when cumulating, will build up stress and will crack and flake off. It is very well known by the amorphous silicon related industry, that periodic cleaning, ideally after every coating run, is mandatory to keep a good yield and to avoid particle contamination of the layer deposition process. PA1 Destruction of the Ozone layer, PA1 global warming. PA1 evacuating the reactor; PA1 generating a glow discharge within the reactor; PA1 feeding a reactive etching gas into the reactor and reacting said etching gas within said reactor; PA1 removing gas with reaction products of said reacting from the reactor, PA1 installing an initial flow of the etching gas into the reactor and reducing the flow thereafter during said reacting. PA1 evacuating the reactor; PA1 generating a glow discharge within the reactor; PA1 feeding a reactive etching gas into the reactor and reacting the etching gas within the reactor; PA1 removing gas with reaction products of said reacting from said reactor and
In Table 1 some examples of known dry etching processes are listed.
TABLE 1 ______________________________________ Etching Radical Solid Out Carrier Comments ______________________________________ SF.sub.6 F Si SiF.sub.4 Preferably with a little Oxygen SF.sub.6 F Si.sub.3 N.sub.4 SiF.sub.4, N.sub.2 Preferably with a little Oxygen NF.sub.3 F Si.sub.3 N.sub.4 SiF.sub.4, N.sub.2 No need for Oxygen CCl.sub.4 Cl Al AlCl.sub.3 Oxygen forbidden C.sub.2 F.sub.6 F, CF.sub.3 SiO.sub.2 SiF.sub.4 Needs ion bombardment ______________________________________
Dry etching is most commonly used to pattern thin films partially protected by a resist film. The resist film is usually deposited by a photolitographic procedure, in particular for manufacturing of integrated circuits.
Nevertheless, the present invention was specifically developed for the application of reactor cleaning of especially PECVD reactors. Nevertheless, the invention can apply to dry etch cleaning of other reactors and to dry etching of workpieces in context with their manufacturing processes. This is particularly true for dry etching applications in the field of wafer processing.
PECVD is used to deposit a thin film on a substrate. A reactive gas is introduced in a low pressure reactor, wherein a glow discharge is sustained. The reactive gas is dissociated and the interesting by-product deposits as a solid film on all the exposed surfaces, thus on the inner surfaces of the reactor itself and on the substrate to be coated. When the desired thickness on the substrate is achieved, the glow discharge is stopped and the coated substrate removed. Thus, at the end of the coating process, not only the substrate is covered but all the reactor wall surfaces exposed directly or indirectly to the glow discharge are coated as well.
The phenomenon that after workpiece coating within a reactor the reactor walls are also coated is not unique for PECVD, but is also known from other coating processes, as e.g from sputtering, evaporation, as by cathodic arc, electron beam or crucible heating.
The typical applications of PECVD, to which the present invention does particularly apply, are related to electronics, optics and opto-electronics. A non-exhaustive list of examples of PECVD film manufacturing and of their application is given in table 2.
TABLE 2 __________________________________________________________________________ Film name Content Starting gas Typical applications __________________________________________________________________________ Amorphous silicon SiH.sub.x SiH.sub.4 Thin Film Transistors, Photodiodes Alloys SiGe, SiC +GeH.sub.4, CH.sub.4 Lower gap or higher gap semicon. Doped a-Si:H SiP.sub.x, SiB.sub.x +PH.sub.3, B.sub.2 H.sub.6 Electric contacts Silicon Nitride SiN.sub.x H.sub.y +NH.sub.3 N.sub.2 Insulator, TFT gate, Barrier Silicon oxide SiO.sub.x H.sub.y SiO(CH.sub.3).sub.x + N.sub.2 O Insulator, TFT gate, Barrier Methylpolysilane Si(CH.sub.3).sub.x SiH.sub.x (CH.sub.3).sub.y Photosens itive resist layer Oxynitride SiNxOy +N.sub.2 O + NH.sub.3 Higher index glass Fluoroxide SiFxOy SiF4 + N2O, etc. Lower index glass __________________________________________________________________________
In the present context one should clearly distinct the coating process, as especially PECVD coating, and the dry etching process, to which the present invention is directed specifically, but in context with previous coating too.
It should be noted that most applications of PECVD films are very sensitive to dust contamination, which is nevertheless not unique for PECVD depositions. Often, and particularly for PECVD processes, the risk of contaminating the film by dust, i.e. any solid contaminants, before, during and after the layer deposition process is to be reduced to the lowest possible level. As shown in Table 2, relevant films deposited by PECVD are silicon-based. Thereby, a corresponding dry etching process for cleaning the reactor should be Fluorine-based. Because the reactor shall not be corroded by such dry etch cleaning, it should resist to Fluorine and is thus made of Fluorine-resisting material, such as of Ni or and especially of aluminum.
If after layer deposition the reactor is not cleaned and subsequently an uncoated substrate is loaded in that uncleaned reactor so as to be coated, there are two annoying consequences:
Such mandatory reactor cleaning is far more conveniently realised by dry etching process than by manual cleaning. Therefore, especially in this field, dry etch cleaning has become most common.
After such a dry etch cleaning, all the deposits due to previous coating deposition, and particularly PECVD deposition, must be removed. Should a thin layer remain even in a remote corner of the reactor, then, when cumulating the cycles, that film layer will also cumulate until some pealing or flaking will generate a severe particle contamination in the reactor. As a consequence, it is very important that the cleaning process lasts until the very last hidden corner of the reactor is accurately cleaned.
During the etching operation, a coating reactor and specifically a PECVD coating reactor is ineffective with respect to coating production. Hence, in order to maximise the equipment throughput, the etch cleaning times should be as short as possible and hence the etching rate should be as large as possible.
Most of the gases involved in dry etching processes are potentially harmful to the environment. Unstable molecules are usually very toxic and corrosive. More stable molecules, if they are released in the environment, can affect the Infra Red transparency of the atmosphere and contribute to global warming, as does fossil fuel combustion (CO.sub.2) and agriculture development (CH.sub.4). Therefore, any industrial processing system should be minimised with respect to rejects to the atmosphere.
Mostly, dry etching systems and especially vacuum reactor cleaning systems do have downstream the processing reactor and the pump arrangement a scrubbing systems, so as to eliminate potentially harmful gases from the exhaust gas mixture.
Thus, the electronic industry is optimizing its processes, not only for the best performance of the devices to be manufactured and not only for the lowest costs thereof, but also for the lowest possible environmental impact of the manufacturing activities.
If industrial gases released to the environment are stable enough to accumulate in the atmosphere, then they can alter the whole equilibrium of the atmosphere. Two well-know examples:
As Ozone destruction is known to be mostly related to Chlorine containing molecules and the present invention is based on the common use of Fluorine etching gases, we concentrate only on global warming.
The driving mechanism of global warming is related to the fact that industrial gas molecules are transparent to the visible incoming spectrum of the sun radiation and are opaque to the Infra Red, outgoing radiation from the earth surface. The normal constituents of the atmosphere, N.sub.2, O.sub.2, Ar, are transparent to the Infra Red. If the atmosphere becomes partially opaque to the Infra Red, typically in the 5 to 20 .mu.m wave length range, global warming takes place.
For a given gas, what defines its contribution to the atmospheric Infra Red opacity, is the product of two key parameters, namely of concentration and of Infra Red absorption cross-section.
The Infra Red absorption cross-section of a given molecule is related to its structure. In short, a molecule is active if it has many non-symmetric chemical bounds. For example SF.sub.6 and C.sub.2 F.sub.6 are active, while F.sub.2 is not active.
The concentration of a given molecule in the atmosphere is a result of two opposite mechanisms: The released amount of this gas in the atmosphere and the destruction of this molecule due to chemical reaction with any constituent of the earth surface. This degradation mechanism is summarised in one parameter, namely the atmospheric life time, giving the time scale for the molecule to be eliminated from the atmosphere. Thus, the concentration key parameter is a function of yearly release and of atmospheric life time.
The atmospheric life time of a molecule is thereby very much related to the reactivity of this molecule. Indeed, if a molecule is very passive, it will survive in the atmosphere for a very long time, and, most likely, it will be very safe in handling, because unable to react with biological molecules. On the contrary, if a molecule is reactive, it will not accumulate in the atmosphere, but will be unsafe, and in the worst case of reactivity even be difficult to keep in storage because of corrosion of storage container and piping.
Therefore, and in view of lowering atmospheric life time, there is a trade of industrial gases used be in favour of moderately chemically active molecules. Indeed, if they are active enough, they will on one hand not risk to build up in the atmosphere, but will raise serious safety handling issues, particularly when handled at high pressure.
With respect to the yearly release as one parameter of IR opacity, it is required that the quantity of released gases remains as low as possible. Combinations of gas scrubber, burner and washer will have to be installed at the exhaust of dry etching process equipment, and increasingly severe thresholds for concentration in rejected gas will be established, in particular for gases with a large potential global warming impact, i.e. of the product of Infra Red cross-section and of atmospheric life time.
Therefore, exhaust gas treatment is becoming a significant fraction of the dry etch processing, and thus significantly contributes to the costs of the equipment. The more etching gas is allowed to flow through the exhaust of a system, the larger will be the load for a downstream gas treatment system.
If the exhaust gases are highly diluted in passive gas, the destruction of the minority component will cost more. The scrubbing system will have to be much larger in volume to keep the necessary reaction time within the scrubber, heat will have to be wasted (and later cooling) for treating all the background gas together with the minority components.
Very reactive molecules are easily destroyed in a scrubbing system. On the other hand, stable molecules are difficult to eliminate. For example CP.sub.4 must be burned in an oxidising atmosphere at a very high temperature in order to be destroyed. It is also very difficult to ensure a very low concentration of CF.sub.4 at the scrubber exhaust.